1. Field of the Invention
The present invention relates to fiber optic probes for spectrophotometry. In particular, the present invention relates to a rugged, mechanically stable fiber optic probe and optical coupler assembly, and to a method for making the probe.
2. Discussion of Background
Recent developments in fiber optics, coupled with the availability of multichannel array-type spectrometers and multiplexing technology, have generated renewed interest in the use of remote spectroscopic techniques for in-line monitoring and process control, environmental monitoring, and medical applications. Signal transmission via optical fibers allows the placement of sensitive equipment in locations remote from industrial process streams, making remote sensing particularly attractive in harsh environments. Multiplexing--the capability of transmitting signals from a plurality of sources to a single instrument--facilitates the efficient use of complex and expensive instrumentation. Optical analysis techniques also improve the quality of the data. Data obtained from a sample is not always truly representative of the source of that sample, since the mere act of taking the sample can alter its properties; frequently, removing a sample can perturb the source as well. Optical analysis techniques can usually be undertaken without removing samples for laboratory analysis elsewhere; therefore, data from optical analyses is frequently more reliable than data obtained by other analytical techniques.
Remote fiber optic probes are essential for in-line monitoring and process control in corrosive and radioactive process environments. In the environmental field, fiber optic probes are used for in situ measurements of fluids in wells, boreholes, storage and process tanks, and so forth. Applications include monitoring groundwater flow, studying the migration of subsurface contaminants, evaluating the progress of remediation operations, and detecting toxic or explosive substances. Fiber optic probes can be used with absorption, diffuse reflectance, and Raman spectroscopy.
The absorbance of a substance is defined as A=-log.sub.10 T, where T=I/I.sub.0, I is the transmitted light intensity, and I.sub.0 the incident light intensity. The absorption spectrum of a substance--the frequency distribution of the absorbance--is used to identify its composition; the amount of light absorbed at different frequencies depends on the concentration of each constituent. Spectrophotometry is the measurement of this absorption spectrum. A typical spectrophotometer includes these basic components: a light source, a probe containing light-transmitting and light-receiving fibers, and a detector. Light from the source is directed to the substance of interest by the transmitting fiber. The light is transmitted through the substance to the receiving fiber and the detector, which produces an output signal proportional to the absorbance of the substance over a range of frequencies. Measurements taken from a suitable reference sample are compared to measurements taken from the test sample to help determine the concentrations of various constituents in the test sample.
Absorption spectroscopy requires samples that are optically translucent or transparent in the range of frequencies being studied. Other techniques based on analysis of the light scattered by the sample, such as diffuse reflectance, fluorescence, and Raman spectroscopy, are useful for in situ analysis of solids or slurries (as used herein, the term "scattered light" includes both elastic (Rayleigh) scattering and inelastic (Raman and fluorescence) scattering). In probes designed for these types of measurements, light is directed to the sample through a transmitting fiber; scattered light is collected by the receiving fiber and returned to the detector. Probes designed for Raman spectroscopy can also be used for fluorescence. For purposes of the following discussion, the terms "Raman spectroscopy" "Raman spectrophotometry," and "Raman measurements" include all forms of inelastic scattering phenomena as well.
Raman spectrophotometry is a sensitive analytical technique based on the inelastic scattering of light (typically, monochromatic light from a laser) by an atom or molecule. While most of the scattered light has the same frequency as the incident light (Rayleigh scattering), a portion is frequency-shifted by an amount equal to one of the resonant frequencies of the molecule. Therefore, in addition to elastically-scattered light having the same frequency as the incident light, the scattered light contains small amounts of light with different frequencies. The pattern of frequency shifts is characteristic of the constituents of the sample; the intensity depends on the concentrations of each constituent in the sample. Raman spectrophotometry provides an excellent indicator, or fingerprint, of the types of molecules present in a sample.
Vibrational and rotational Raman spectra are typically in the visible or near-infrared (NIR) region, therefore, Raman spectra are less severely attenuated than infrared (IR) absorption spectra by transmission over optical fibers. Therefore, Raman spectrophotometry can be done with normal silica fiber optic cables instead of the more expensive and fragile types of fibers needed for IR absorption spectrophotometry. In addition, Raman spectrophotometry is particularly useful for identifying the constituents of a substance since Raman spectra generally contain more spectral lines--and sharper lines--than other types of spectra.
A problem encountered in Raman spectrophotometry is the small scattering cross section, that is, the very low intensity of the Raman-scattered light compared to the intensity of the incident light (also termed the "exciting light"). Like absorption spectroscopy, Raman spectrophotometry requires a light source, an optical probe with light-transmitting and light-receiving fibers (also termed exciting and collecting fibers, respectively), and a detector. In addition to Raman-scattered light, some of the exciting light and some elastically-scattered light are reflected back to the receiving fiber. Light may also be reflected to the receiving fiber by the interior surfaces of the probe. In addition, monochromatic light transmitted by an optical fiber excites the fiber molecules, causing fluorescence and Raman scattering within the fiber itself. This "self-scattering" or "silica scattering" generates a signal that interferes with the Raman signal collected from the sample of interest.
When making Raman measurements with fiber optics, it is therefore necessary to reduce the amounts of nonshifted sample-induced scattered and reflected light returning to the spectrometer, as well as reduce fluorescence and silica Raman scattering generated in the fibers themselves. To filter out this noise, light from the transmitting fiber may be directed through a narrow bandpass filter at the fiber tip that transmits the laser frequency but rejects signals arising from the fiber (known as fluorescence and silica scattering) and extraneous light from the laser source (such as plasma lines, fluorescence, or superluminance). Light returning through the receiving fibers passes through a long-pass optical filter that rejects elastically-scattered light and reflected laser light but transmits Raman signals from the sample. High-intensity laser sources and sensitive detectors with high light gathering power and high stray light rejection are needed to isolate and measure the low intensity Raman signal due to the sample. Chemometric techniques are also used to help factor out background noise and identify the signal of interest. Instrumentation for Raman spectrophotometry is costly and delicate, requires high-precision, high-maintenance optical components, and is not well suited for use in many industrial process environments.
Presently-available fiber optic Raman probes include a probe having slanted tips (McLachlan, et al., U.S. Pat. No. 4,573,761). The transmitting fiber is surrounded by a plurality of receiving fibers spaced about the axis of a cylindrical housing which is closed at one end by a transparent window. The receiving fibers converge along lines which intersect at a point that is adjacent to or beyond the outer surface of the window. A Raman probe for light scattering measurements may include fibers with angled endfaces, such as the probe described in U.S. Pat. No. 5,402,508 issued to O'Rourke, et al., the disclosure of which is incorporated herein by reference. A variety of fiber optic probes, including Raman probes, are described by S. E. Nave, et al. in "Sampling probes enhance remote chemical analyses," Laser Focus World, December, 1995 (incorporated herein by reference). Several commercially available probes include efficient dual-fiber designs incorporating micro-optics, beamsplitters, and filters at the probe tip (such probes are available from Dilor Instruments SA (Edison, N.J.) and EIC Laboratories (Norwood, Mass.)).
A variety of closely packed multifiber arrays and processes for making such arrays are known in the art. Le Noane, et al. (U.S. Pat. No. 5,519,801) describe multicore optical guides wherein the optical fibers are very accurately positioned with respect to one another and with respect to the external contours of a matrix. Kuder, et al. (U.S. Pat. No. 5,222,180) place a bundle of polymer optical fibers into a rigid sleeve that has a softening point higher than the softening point of the fibers. When the resulting assembly is heated, the fibers expand, resulting in a close-packed geometry.
Ekinaka, et al. (U.S. Pat. No. 4,173,392) and Bazinet, Jr., et al. (U.S. Pat. No. 3,681,164) use bonding agents to hold a plurality of fibers in place. The Ekinaka, et al. glass fiber light guide consists of an elongated bundle of generally parallel glass fibers embedded in a hardened resin matrix, sheathed by a pair of thin protective layers. Bazinet, Jr., et al. apply a ceramic-based bonding agent to a bundle of fiber optic tips, then pull the bundle back into a plug until the tips are flush with the end.
Other methods for holding fibers in place include heat-shrinkable tubing and cords. Jones (U.S. Pat. No. 3,586,562), Hicks, Jr. (U.S. Pat. No. 3,224,851), and Phaneuf, et al. (U.S. Pat. No. 3,198,059) use heat shrinkable plastic tubing to secure the ends of a fiber bundle in place. Sheldon ties a cord about the ends of a bundle, then dips the ends in a cohesive liquid agent to permanently fix the ends in position (U.S. Pat. No. 3,301,648). The resulting bundle can be covered by heat-shrink tubing. Kapany makes a multifiber bundle by aligning a plurality of glass rods within a tube, then draws the resulting structure to the desired diameter (U.S. Pat. No. 3,190,735).
Multifiber couplers are used in many fiber optic devices. In U.S. Pat. Nos. 5,289,056 and 5,058,985, Davenport, et al. disclose an optical coupler that includes a housing and a plurality of optical fibers, the output ends of which are distributed to various spaced-apart locations. The input ends of the fibers are stripped of cladding, then compressed together within an inner sleeve so that boundaries between the individual ends are substantially eliminated. To protect the input ends from damage by high brightness light sources, a light-transmissive rod with a higher thermal coefficient than the fibers is disposed in optical contact with the input ends. Coutandin, et al. and Xu also use heat-shrinkable tubing in their devices. Coutandin, et al. (U.S. Pat. No. 5,185,832) make an optical coupler by bundling a plurality of polymer optical waveguides inside a plastic tube, pushing a heat-shrinkable sleeve over the tube, and heating to a temperature at which the sleeve contracts. Xu (U.S. Pat. No. 4,923,268) uses fibers that have a heat fusing temperature in a range achievable by exterior heating of a shrink sleeve. In his device, the fibers are fused together along a limited length within the sleeve. O'Rourke, et al. provide a Raman probe having a spring-loaded filter assembly, wherein the close proximity of the filter assembly to the probe tip minimizes self-scattering generated by the optical fibers (U.S. Pat. No. 5,710,626).
Presently-available Raman probes depend on precision optical components and single-strand optical fibers for excitation and light collection. Assembly of such probes requires high-precision machining and stringent optical alignment procedures. Conventional techniques for alignment the filters and optical fibers--and maintaining them in alignment--are difficult and time-consuming to implement. The probes are expensive and delicate, rendering them unsuitable for most field installations. In part because of these problems, in part because other needed instruments were large, complex, and expensive, Raman spectrophotometry has historically been confined to research laboratories.
Our co-pending application entitled "Fiber Optic Probe" (Ser. No. 08/676,432, filed Jul. 8, 1996), the disclosure of which is incorporated herein by reference, addresses these problems. In this application we describe a fiber optic probe and optical coupler assembly for light scattering measurements, and a method for making the assembly and aligning a plurality of optical fibers therein. The probe includes a probe body with a window across its tip for protecting the interior, at least one light-transmitting fiber, at least one light-receiving fiber, and (if desired), in-line devices such as filters and lenses positioned in optical communication with the fibers. The coupler maintains the relative alignment of the fibers, which can be cut to install filters and other in-line devices. The design of the coupler allows the cut ends to be re-aligned quickly and accurately without the need for time-consuming procedures or costly precision alignment equipment. The probe is simple, rugged, requires no high-precision machining or optical alignment procedures in assembly, and is economical to manufacture.
Despite the availability of numerous designs for fiber optic probes, including that described in application Ser. No. 08/676,432, there is a continuing need for simple, rugged, inexpensive, easy-to-manufacture and easy-to-align probes for light scattering measurements (including but not limited to Raman measurements). The increased availability of such probes would further the use of Raman spectrophotometry and other optical techniques for on-line monitoring in a wide range of laboratory, medical, environmental, and industrial environments.